In the commercial nondestructive inspection of metallic tubular goods by the conventional eddy current method, an excited eddy current probe comprised of one or more input or primary windings and one or more output or secondary windings is passed along the surface of the tubular member being inspected. The output signal from the eddy current probe is monitored to determine a change in the phase angle of the probe output signal. Such a change provides an indication of an anomaly in the wall of the pipe or a change in the wall thickness of the pipe. In practice, the windings of the probe are spaced from the surface of the tubular member by some slight distance. Either or both the probe and the tubular member are moving and, as a practical matter, it is virtually impossible to prevent the spacing between the windings and the surface of the tubular member from changing. This changing separation is known as lift-off and causes the output signal from the probe to change in both magnitude and phase. It has long been a major effort in the art of eddy current inspection to provide effective methods and apparatus for eliminating the effect of probe lift-off from the eddy current inspection signals.
I have been able to achieve the above objective by the use of relatively simple, inexpensive, and reliable means that is easy to use and readily adapted for commercial operations.